Abstract
The fine tuning of hormone (e.g., auxin and gibberellin) levels and hormone signaling is required for maintaining normal embryogenesis. Embryo polarity, for example, is ensured by the directional movement of auxin that is controlled by various types of auxin transporters. Here, we present pieces of evidence for the auxin-gibberellic acid (GA) hormonal crosstalk during embryo development and the regulatory role of the Arabidopsis thaliana Calcium-Dependent Protein Kinase-Related Kinase 5 (AtCRK5) in this regard. It is pointed out that the embryogenesis of the Atcrk5-1 mutant is delayed in comparison to the wild type. This delay is accompanied with a decrease in the levels of GA and auxin, as well as the abundance of the polar auxin transport (PAT) proteins PIN1, PIN4, and PIN7 in the mutant embryos. We have previously showed that AtCRK5 can regulate the PIN2 and PIN3 proteins either directly by phosphorylation or indirectly affecting the GA level during the root gravitropic and hypocotyl hook bending responses. In this manuscript, we provide evidence that the AtCRK5 protein kinase can in vitro phosphorylate the hydrophilic loops of additional PIN proteins that are important for embryogenesis. We propose that AtCRK5 can govern embryo development in Arabidopsis through the fine tuning of auxin-GA level and the accumulation of certain polar auxin transport proteins.
Highlights
Embryogenesis is the first stage of the plant life cycle, which initiates with fertilization and zygote development and it terminates with the maturation of the embryo
We found that the progression of embryogenesis was delayed in the Atcrk5-1 mutant having reduced gibberellic acid (GA) content and shorter shoot apical meristem (SAM)-root apical meristem (RAM) distance in late embryo stages
We found that the AUX1::YFP expression level was decreased in the Atcrk5-1 mutant in all of the embryo developmental stages in comparison to the wInitl.dJ.tMypole. .ScMi. 2o0r1e9o, 2v0e,r6,12th0 e distribution of the AUX1::YFP protein was more restricted at the root12poofle23 in the mutant background
Summary
Embryogenesis is the first stage of the plant life cycle, which initiates with fertilization and zygote development and it terminates with the maturation of the embryo. Until the 16-cell stage, PIN7 is located in the apical membranes of the suspensor cells, ensuring auxin transport from the suspensor to the pro-embryo, while PIN1 is localized in the pro-embyo in a non-polarized manner. Members of the AUX1/LAX family were shown to be redundantly required for correct cell organization in the radicle tip of mature embryos [24]. These data indicate that the auxin efflux and influx carriers collaborate for regulating cell specification, and both types of transporters are important for well-balanced auxin transport that is responsible for embryo polarity determination and normal embryo development [17]
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